Positron propagation in semi-relativistic plasmas: particle spectra and the annihilation line shape

By solving the Fokker-Planck equation directly we examine the effects of annihilation, particle escape and injection on the form of a steady-state positron distribution in thermal hydrogen plasmas with kT < mec. The positron fraction considered is small enough, so it does not affect the electron distribution which remains Maxwellian. We show that the escape of positrons in the form of electron-positron pairs and/or pair plasma, e.g. due to the diffusion or radiation pressure, has an effect on the positron distribution causing, in some cases, a strong deviation from a Maxwellian. Meanwhile, the distortion of the positron spectrum due to only annihilation is not higher than a few percent and the annihilation line shape corresponds to that of thermal plasmas. Additionally, we present accurate formulas in the form of a simple expression or a one-fold integral for energy exchange rates, and losses due to Møller and Bhabha scattering, e+e−-, eeand ep-bremsstrahlung in thermal plasmas as well as due to Compton scattering in the Klein-Nishina regime. Suggesting that annihilation features observed by SIGMA telescope fromNovaMuscae and the 1E 1740.7–2942 are due to the positron/electron slowing down and annihilation in thermal plasma, the electron number density and the size of the emitting regions have been estimated. We show that in the case of Nova Muscae the observed radiation is coming from a pair plasma stream (ne+ ≈ ne− ) rather than from a gas cloud. We argue that two models are probably relevant to the 1E 1740.7–2942 source: annihilation in (hydrogen) plasmane+ <∼ ne− at rest, and annihilation in the pair plasma stream, which involves matter from the source environment.